To ensure that future statements run under releases prior to
2.1 at least yield runtime exceptions (the import of __future__ will
fail, because there was no module of that name prior to 2.1).

To document when incompatible changes were introduced, and when they will be
— or were — made mandatory. This is a form of executable documentation, and
can be inspected programmatically via importing __future__ and examining
its contents.

Each statement in __future__.py is of the form:

FeatureName=_Feature(OptionalRelease,MandatoryRelease,CompilerFlag)

where, normally, OptionalRelease is less than MandatoryRelease, and both are
5-tuples of the same form as sys.version_info:

(PY_MAJOR_VERSION,# the 2 in 2.1.0a3; an intPY_MINOR_VERSION,# the 1; an intPY_MICRO_VERSION,# the 0; an intPY_RELEASE_LEVEL,# "alpha", "beta", "candidate" or "final"; stringPY_RELEASE_SERIAL# the 3; an int)

OptionalRelease records the first release in which the feature was accepted.

In the case of a MandatoryRelease that has not yet occurred,
MandatoryRelease predicts the release in which the feature will become part of
the language.

Else MandatoryRelease records when the feature became part of the language; in
releases at or after that, modules no longer need a future statement to use the
feature in question, but may continue to use such imports.

MandatoryRelease may also be None, meaning that a planned feature got
dropped.

Instances of class _Feature have two corresponding methods,
getOptionalRelease() and getMandatoryRelease().

CompilerFlag is the (bitfield) flag that should be passed in the fourth
argument to the built-in function compile() to enable the feature in
dynamically compiled code. This flag is stored in the compiler_flag
attribute on _Feature instances.

No feature description will ever be deleted from __future__. Since its
introduction in Python 2.1 the following features have found their way into the
language using this mechanism: